1. Field of the Invention
The present invention relates to a capacitance type sensor suitably used for detecting a force applied from outside.
2. Description of the Related Art
A capacitance type sensor is in general use as a device for converting magnitude and direction of a force applied by an operator into electric signal. For example, a device having the capacitance type sensor for inputting operation of multidimensional direction incorporated as a so-called joystick is used as an input device of a mobile phone.
A capacitance type sensor can be used to input an operation having a specified dynamic range as a magnitude of a force applied by an operator. It can also be used as a 2-dimensional or 3-dimensional sensor capable of dividing an applied force into all dimensional components, for force detection. Among others, a capacitance type force sensor having a capacitance element formed by two electrodes to detect an applied force on the basis of changes of capacitance values caused by variations of distance between the electrodes is now in practical use in a variety of fields in terms of the merit that the structure can be simplified to reduce costs.
Reference is herein made for example to International Application No. PCT/JP00/09355 under the Patent Cooperation Treaty filed on Dec. 27, 2000 by the same applicant. It describes a capacitance type sensor 501 as shown in FIG. 13. The capacitance type sensor 501 has a substrate 520, a detective member 530 which is an operating member to which a force is applied from outside by a user, a displacement electrode 512 having conducting properties, capacitance element electrodes E501-E505 and a reference electrode (a common electrode) E500 formed on the substrate 520, an insulating film 513 formed to be laid over the substrate 520 so as to be closely contacted with the capacitance element electrodes E501-E505 and the reference electrode E500, and a supporting member 560 for fixedly supporting the detective member 530 and the displacement electrode 512 on the substrate 520.
As shown in FIG. 14, the capacitance element E505 of a circular form with center at the origin O, the capacitance element electrodes E501-E504 of a sector form arranged around it, and the reference electrode E500 of an annular form with center at the origin O arranged further around them are formed on the substrate 520. The capacitance element is formed between the displacement electrode 512 and the respective capacitance element electrode E501-E505.
In this capacitance type sensor 501, signals such as clock signals are input to the capacitance element electrodes E501-E505. When the detective member 530 is displaced by a force applied from outside in the condition in which the signals are being input to the capacitance element electrodes E501-E505, the displacement electrode 512 is displaced in a Z-axis direction in response to the displacement. Then, the distances between the electrodes of the capacitance elements formed between the displacement electrode 512 and the respective capacitance element electrodes E501-E505 vary and thereby the capacitance value of those capacitance elements change as the variations of the distance between the electrodes, to thereby produce a phase lag in the signals input to the capacitance element electrodes E501-E505. By using this phase lag in the input signals, the displacement of the detective member 530, i.e., the magnitude and direction of the force applied from outside to the detective member 530 in directions of X-axis, Y-axis, and Z-axis, are detected.
However, since the capacitance type sensor 501 is constructed so that the detective member 530 and the displacement electrode 512 are in contact with each other (integral with each other), when the detective member 530 is displaced, the displacement electrode 512 is also displaced automatically. Due to this, if a returned position of the detective member 530 after operation differs from an initial position of the same before operation, the returned position of the displacement electrode 512 will also vary. As a result, the output signals from the capacitance type sensor 501 before operating the detective member 530 and those from the same sensor after operating the detective member 530 vary. This results in increase in hysteresis of output signal from the capacitance type sensor 501 and thus reduction in reproducibility as the sensor. Also, if the detective member 530 is displaced only a little against will of an operator, then the displacement electrode 512 will also be displaced automatically to easily introduce errors (malfunctions) of the sensor.